FIG, 1 illustrates an image capture system, such as a camera, including an imaging optics 10 having a focal length, f and an imaging sensor 12, wherein the focal length is a property of the imaging optics 10. An object 14 at a distance do in front of the camera will be properly focused at the imaging sensor 12 at a distance di behind the imaging optics 10 if the relationship between do, di, and f is
                                          1                          d              o                                +                      1                          d              i                                      =                  1          f                                    (                  Equation          ⁢                                          ⁢          1                )            
If the camera-to-object distance changes to a distance do±δo in front of the camera, as illustrated in FIG. 2, then the image will be out of focus by a distance δi given by
                              δ          i                =                  ±                                    d              i                        (                          1              -                              1                                  1                  +                                                            d                      i                                                              d                      o                                                        -                                                            d                      i                                                                                      d                        o                                            ±                                              δ                        o                                                                                                                  )                                              (                  Equation          ⁢                                          ⁢          2                )            Similarly, the image will be out of focus by δi, if the focal length of the camera changes such that the focal plane is moved to a distance di-δi behind the imaging optics. No adverse effect to image quality will be perceived, if δi is within the depth of focus of the image capture system, given by
                              δ          i          ′                =                              ±            2                    ⁢                                          ⁢                                    λ              ⁡                              (                                  f                  D                                )                                      2                                              (                  Equation          ⁢                                          ⁢          3                )            where D is the entrance aperture diameter of the image capture system. The corresponding depth of field δ′o is given by
                              δ          o          ′                =                              ±            2                    ⁢                                          ⁢                                    λ              ⁡                              (                                                      d                    o                                    D                                )                                      2                                              (                  Equation          ⁢                                          ⁢          4                )            hence objects within the range of do±δ′o will not appear out of focus.
The correct focus of an image capture system can change over time due to dynamic changes to the system, such as those caused by the thermal environment. When the focal length changes the depth of field by more than δ′or the camera-to-object distance changes more than δ′o, it is necessary to refocus the image capture system. Many methods for correcting the focus of the imaging system have been disclosed in prior art, with most methods requiring additional optics and sensors. Scene-based methods have the advantage that focus is determined by analyzing the image data collected of the scene, usually using the same optics and detector used to acquire the in-focus images.
Prior scene-based methods for determining the focus have been proposed that compare the sharpness between image data formed at various focus positions. The focus method disclosed in U.S. Pat. No. 4,183,642 by Fukuoka, on Jan. 15, 1980, titled “AUTOMATIC FOCUSING DEVICES,” finds a focus position that maximizes the contrast difference between adjacent pixels as the focus changes between images. Many scene-based methods assume that the plurality of images collected at different focus positions do not shift, rotate, or change in scale; thus the only change between the images on a pixel-per-pixel basis is the change in focus. If the imaging system moves between images, then these methods with such an assumption do not work well, unless the plurality of images are spatially registered to sub-pixel accuracy.
The relative shift between images can be avoided by imaging the same scene multiple times at different focus positions, simultaneously, through the same boresight to determine focus. One such method proposed in U.S. Pat. No. 5,166,506 by Fiete et al. on Nov. 24, 1992, titled “METHOD FOR DETERMINING AN IMAGING SYSTEM FOCUS ERROR,” uses beam splitters to image the scene onto a plurality of sensors. This method, however, requires additional optical elements and the beam splitters reduce the signal arriving at each sensor. The method proposed in U.S. Pat. No. 6,023,056 by Fiete et al. on Feb. 8, 2000, titled “SCENE-BASED AUTOFOCUS METHOD,” correlates a step function with the edges in the scene at various focus positions, thus it is insensitive to changes in scale, rotation, and position; but the location of the edges are usually not known, hence, requiring the calculation of the edge correlation for all of the image data, which can be computationally intensive for large images.
There is a need therefore for a scene-based focus method that is not computationally intensive, does not require precise alignment or spatial registration of a plurality of images collected at different focus positions, and does not require additional optical elements in the design of the image capture system.